AREVA the Testing of Transformer

Transformer

AREVA T&D Enerji Endüstrisi

A.Ş.

Barış Mah. E 5 Altı, 1801 Sok. No 104 41410 Gebze- Kocaeli / TÜRKİYE Tel. : + 90 262 648 33 00

Fax : + 90 262 641 20 36

Page : 2

THE TESTING OF POWER TRANSFORMERS

PREFACE Test procedures and equipment used for the testing and measurement of power transformers at AREVA Gebze factory are dealt in the scope of this booklet. The electrical characteristics and dielectric strength of the transformers are checked by means of measurements and tests defined by standards (e.g. TS, IEC, DIN/VDE, ANSI, NEMA, BS….. etc.) and by the requirements of customers specifications. Summary of the tests and measurements processes are given as follows:

ROUTINE TESTS: Pages 1. Measurement of voltage ratio and check of vector relationship 2. 3. 4. 5. 6. 7. 8. 9.

Measurement of winding resistance Measurement of impedance voltage and load loss Measurement of no-load loss and current Dielectric tests Separate-source voltage withstand test Induced over-voltage withstand test Partial discharge measurement Test on on-load tap changer

3 6 8 11 13 15 16 19 22

TYPE TESTS AND SPECIAL TESTS 10. 11. 12. 13. 14. 15. 16. 17.

Temperature rise test Measurement of zero-sequence impedance Measurement of voltage and current harmonics Measurement of insulation resistance Measurement of capacitance and tan δ Lightning impulse test Switching impulse test Measurement of acoustic sound level

23 27 29 30 31 32 36 38

List of tests and measuring equipment of the test field

Prepared by : Haluk ODOGLU

41

March 2004 ( 5. Edition )

All rights of this document are reserved to AREVA T&D Enerji Endüstrisi A.Ş. To duplicate and to pass over to third persons without previous permission is forbidden. This document is subject to change.

Page : 3

THE TESTING OF POWER TRANSFORMERS

1. MEASUREMENT OF VOLTAGE AND CHECK OF VECTOR RELATIONSHIP The voltage ratio (i.e. turn ratio) of the transformer is the ratio of voltages at no-load. Purpose of the measurement: verification of no-load voltage ratios specified by the specification and detection of any problem within the coils or tapping connections. Measurements are carried out on all taps and on all phases. Measuring Circuit Turn ratio measurement is performed by two separate methods; 1. measurement by bridge method 2. measurement of voltage ratios 1. Turn ratio measurement is carried out by means of a voltage ratio measuring bridge in one-phase basis between the winding pairs. Measurements are repeated at all phases and all taps. During measurement only the turn ratio between the windings in which same magnetic flux flows. In other words, the turn ratio can be measured between the winding pairs, which are in parallel in vector diagrams (figures 1-1, 1-2, 1-3). The supply voltage is 220 V a.c. and error of the bridge is less than ± 0,1%. 1 2

~

U1

220 V

U2

3

d - The transformer with adjustable taps

c - Transformer to be measured e - Zero indicator

U1 – The supply voltage of the bridge and H.V. winding (220 V, 50 Hz) U2 – The induced voltage in L.V. winding Figure 1-1: Measurement of voltage ratio measuring bridge

Theoretical turn ratio =

the voltage of HV winding the voltage of LV winding

The theoretical turn ratio of the transformer is adjusted on the tapped transformer of the bridge. ‰ error indicator knob is adjusted until the balance is reached in the zero indicator. The reading from the error indicator scale shows the difference (deviation) between the actual turn ratio and turn ratio in ‰.

Deviation =

( measured

turn ratio ) − ( theoretical turn ratio ) ⋅ %100 theoretical turn ratio

2. Voltage ratio measurements are generally performed by the digital instruments produced for this purpose. In addition to voltage ratio measurement, the determination of the vector group All rights of this document are reserved to AREVA T&D Enerji Endüstrisi A.Ş. To duplicate and to pass over to third persons without previous permission is forbidden. This document is subject to change.

Page : 4

THE TESTING OF POWER TRANSFORMERS

(connection group) and current measurements can be performed with these instruments, it is necessary that the instrument must have 3-phase system for vector group determination. The method of comparing the voltages of dual vectors enables the measurement of phase shifting between the vectors. The deviation in the turn ratio shall be < ± 0,5 %. Vector Group In multiphase transformers, primary and secondary connections can be either star (Y), delta (D) or zigzag (Z), depending on the type of the transformer. The phase angle between the primary and secondary windings changes between 0° and 360°. In vectorial denotation, when H.V. windings shows 12 (0), the numbers of the other windings in the connection group show the number of the clock in comparison with real or imaginary neutral point. For example, in Dyn5 connection group H.V. winding is Delta (D), L.V. winding is Star (Y) and there is a phase displacement of 150 (5x30°) between the windings. When the vector of H.V. shows 12 (0), the vector of L.V. shows 5 (150° lag). The connection group is defined only for three-phase transformers. In connection group denotation, the H.V. winding is shown first (as a reference) than the other windings are as followed up. The vector diagrams is also checked at the same time. The correct connection of the measurement cables between the transformer and between the bridge verifies the vector relationship, otherwise it is not possible to balance the bridge. Besides the above mentioned, the check of vector relationship and the check of polarity also could be done using a voltmeter. In this method AC or DC voltages could be applied. The wiring connections related with the AC method are given in standards as in details. An example to this method is illustrated in a phase diagram as below: B

Example : Vector group: Dyn5 Measuring procedure:

a

1- three phase voltage is applied to ABC phase 2- the voltage between any two phases is measured ( e.g. AB) 3- A and n terminals are short-circuit 4- the voltage between B and a′ is measured 5- the voltage between C and c′ is measured c´ Figure 1-2: Measuring of vector group

c n

a´

b

A

C

n´ b´

As it can be seen from the phase diagram, to obtain Dyn5 vector group the following condition should be realised: C c’ > AB > B a’ The other vector relationships can be checked by using the same principles.

All rights of this document are reserved to AREVA T&D Enerji Endüstrisi A.Ş. To duplicate and to pass over to third persons without previous permission is forbidden. This document is subject to change.

Page : 5

THE TESTING OF POWER TRANSFORMERS

Figure 1-3: Connection symbols for three-phase transform All rights of this document are reserved to AREVA T&D Enerji Endüstrisi A.Ş. To duplicate and to pass over to third persons without previous permission is forbidden. This document is subject to change.

Page : 6

THE TESTING OF POWER TRANSFORMERS

2. MEASUREMENT OF WINDING RESISTANCE Although, the winding resistance values are not the guaranteed values given to the customers, they are needed in connection with the load loss measurement when the load losses are corrected to correspond to the reference (e.g. 75°C) temperature. The resistance measurement will also show whether the winding joints are appropriate and the windings are correctly connected. The winding resistances that vary with the temperature strongly, are the ohmic/d.c. resistance’s of a winding and the resistance is computed as follows:

R2 = R1 ⋅ where;

235 + t 2 235 + t1

( for copper )

R2 = R1 ⋅

225 + t 2 225 + t1

( for aluminium )

R2 = winding resistance at t2 temperature R1 = winding resistance at t1 temperature

Therefore, wherever the winding resistances are stated, the temperatures during the measurement must be given. The resistances between all pairs of phase terminals at all tapping connections are measured. During the measurement of the resistance, winding temperature should be correctly measured. Direct current can be obtained from a constant-current supply or from a battery unit. The value of the direct current should be high enough to ensure correct measurement and should be low enough to prevent any effects on the winding temperature. In practise this value should be greater than 1,2 x I0 and less than 0,1x In. The time constant of the measurement circuit will depend on the ratio of L/R. When the test object is assumed to be composed of a R resistance and L inductance which is series connected to it, U voltage applied to this circuit will be; R − t U i = (1 − e L ) R

where time coefficient depends on L/R ratio.

If the measuring current increases, it leads to the reduction of inductance due to saturation of the core, these enables the current to reach the steady state condition in a short time. After switching on the supply voltage to the measurement circuit, it should be waited until the current becomes stationary, otherwise measurement errors will be occurred. Measurement Circuit Winding resistances can be measured by any of the following methods, by current-voltage method or by bridge method. Measuring sensitivity can be increased by using the digital measuring instruments. The circuit of the measurement by current-voltage method is given in figure 2-1. In the current-voltage measuring method, by applying the winding current through the reference resistance in the system, the voltage drops occurred in both resistances. This voltage drop values of reference and winding resistances are compared to determine the value of unknown resistance (winding resistance) which can be read directly from the bridge instrument. It is necessary to care, in older avoid of very high voltages during the switching on and off the circuit, the voltmeter shall not be kept in the circuit during this time.

All rights of this document are reserved to AREVA T&D Enerji Endüstrisi A.Ş. To duplicate and to pass over to third persons without previous permission is forbidden. This document is subject to change.

Page : 7

THE TESTING OF POWER TRANSFORMERS

a

S

A

A

B

b

C

c N

V

n

Figure 2-1: Winding resistance measurement by Current-Voltage method In the bridge method measurement, the principle is the comparison of unknown resistance with a known resistance. This will be accomplished to make the current flowing through Galvanometer to zero by bringing the arms of the bridge into the equilibrium. The lower resistances with ( 72,5 kV this transformers shall all, if not otherwise agreed, be tested with partial discharge measurement. The partial discharge performance shall be controlled according to the time sequence for the application of the voltage as shown in figure 7.2 :

U 2 = 1,3 ⋅ U m / 3

phase – earth and U2 = 1,3 . Um phase - phase

A : 5 min B : 5 min C : test duration D : ≥5 min E : 5 min Um : Highest voltage for

C D

B

E

A

equipment

U2 1,1 ⋅ U m /

U1

U2

3

1,1 ⋅ U m / 3

time

Figure 7.2: Test duration voltage – time curve b) non-uniformly insulated windings : For three phase transformers, two test sets are required ; 1. A phase-to-earth test with rated withstand voltages between phase and earth with partial discharge measurement. 2. A phase-to-phase test with earthed neutral and with rated withstand voltages between phases with partial discharge measurement. The test shall be carried out in accordance with uniformly windings ( subclause a ). On single –phase transformers, only a phase-to-earth test is required. The test sequence for three phase transformer consist of three single-phase applications of test voltage to the individual phases with different points of the winding connected to earth at each time. At this type of windings, induced overvoltage test and separate source voltage withstand test ( at the pase terminals ) are performed in the same time. For the three single-phase tests for the phase-to-earth insulation ; U 2 = 1,5 ⋅U m /

3

For the partial discharge performance evaluation, during the phase-to-phase test, measurements should be taken at U2 = 1,3 . Um . For Um = 420 kV and 550 kV transformers with test values of 460 kV and 510 kV, the PD evaluation level should be reduced to U2 = 1,2 . Um during the phaseto-phase test and U 2 = 1,2 ⋅ U m /

3 during the phase-to-earth test.

All rights of this document are reserved to AREVA T&D Enerji Endüstrisi A.Ş. To duplicate and to pass over to third persons without previous permission is forbidden. This document is subject to change.

Page : 18

THE TESTING OF POWER TRANSFORMERS

Test circuit

C

W G 3~

1- synchronous supply gen. 2- test transformer 1 3- current transformer and ammeter 4- voltage transformer and voltmeter 5- transformer under test 6- Capacitive voltage divider

2

V

c b

B

U

a

A

N

A 3

5

N

V 4

6

V

Figure 7.3: Test circuit for induced over-voltage withstands test on non-uniformly insulated winding of a three-phase transformer The test circuit given in fig. 7.3 is for the transformer which HV neutral point is insulated according to 1/3 of the test voltage.

Long duration induced AC voltage test ( ACLD ) : Uniformly and non uniformly insulated windings. Three- phase transformer shall be tested either phase-to-phase in a single-phase connection or in a symetrical three-phase connection. The neutral terminal ( if present ) of the winding under test shall be earthed. The other separate windings, if they are star-connected they shall be earthed at the neutral, and if they are delta-connected they shall be earthed at one of the terminals or earthed through the neutral of the supplying voltage source. The duration and the voltage levels are given in fig. 7-4. 5 min 5 min test duration for Um>300 kV 60 min for Um